1. Field of the Invention
The invention relates to phase change switches and other control elements or devices, and more particularly, to phase change switches or control devices having a dynamic range of impedance, and circuits and components employing such switches or control devices. More specifically, the invention relates to such switches which can be employed in circuits such as on frequency selective surface arrays, for controlling current flow throughout the array, through the use of the switches. By controlling such current flow, the properties of the frequency selective surface array can be actively controlled. In addition, the invention also relates to implementation of such switches and other control devices in circuits, and the circuits themselves, that use conductive structures and dielectrics to guide electromagnetic (EM) waves.
2. Background of the Invention
Mechanical on/off switches have been used in circuits designed to interact with electromagnetic waves, and in particular, circuits designed to handle guided electromagnetic (EM) waves. Another set of such applications includes two-dimensional periodic arrays of patch or aperture elements known as frequency selective surfaces (FSS), the capabilities of which have been extended by addition of active devices, such as switches, and which are generally known as active grid arrays.
The mechanical process in these on/off switches involves the physical motion of a conductor (the “bridge”) between two positions, i.e., one where the bridge touches another conductor and completes the direct current (DC) conducting path of the circuit (“closed”) or moves close enough to it that the capacitive impedance is low enough to complete the path for alternating current (AC) flow, and the other where it has moved away from the contact (“open”) to break the DC conducting circuit path or to raise the capacitive impedance to block AC flow. Such mechanical switches have been made at micrometer size scale in so-called MEMS—Micro-Electro Mechanical Systems. MEMS switch technology to date has shown poor lifetimes and packaging costs.
A key goal in the use of MEMS switches with guided EM waves in the so-called radio frequency (RF) bands is to provide controllable phase delays in a circuit. This is done by using a set of switches to introduce combinations of fixed length phase delay branches into a circuit path. The degree of phase delay control is related to how many separate branches (and switches to control them) are added to the circuit. The switching in or out of a given fixed delay branch provides a step change in the net circuit phase delay. In this approach, if finer steps are desired to cover the same range of total phase delay, then more branches and switches are required.
Alternatively, transistor and transistor-like semiconductor switching devices have been used in circuits designed to interact with electromagnetic waves and in particular, in circuits and components thereof that guide EM waves. Such devices which include PIN diodes and field effect transistors (FETs) form the basis of a collection of solid-state circuits operating on guided EM waves of up to gigahertz (e.g., GHz, 1 GHz≈109 Hz) for use in microwave and communication systems. However, for the specific applications herein, the semiconductor switching devices typically have shortcomings in several areas, i.e., GHz and above. Such shortcomings may include high switching power required or high insertion losses.
In the field of semiconductor memory devices, it has been proposed to use a reversible structural phase change (from amorphous to crystalline phase) thin-film chalcogenide alloy material as a data storage mechanism and memory applications. A small volume of alloy in each memory cell acts as a fast programmable resistor, switching between high and low resistance states. The phase state of the alloy material is switched by application of a current pulse, and switching times are in the nanosecond range. The cell is bi-stable, i.e., it remains (with no application of signal or energy required) in the last state into which it was switched until the next current pulse of sufficient magnitude is applied.